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Energy, exergy and economic analysis are proposed herein to evaluate regasification systems in Floating Storage Regasification Units (FSRUs). Three regasification systems typical in these types of vessels are considered: seawater system, open loop propane system and closed loop water-glycol system. The energy and exergy analyses were performed using the Engineering Equation Solver (EES), while Suite AspenONE programmes were used for the economic assessment. The exergy analysis provides a better understanding of the components of physical exergy (thermal and mechanical exergy) in order to define an exergy efficiency applicable to any liquefied natural gas (LNG) regasification system or FSRU. The results obtained prove the seawater regasification system to be most efficient from an energy and exergy standpoint. The specific energy consumption and exergy efficiency for this system are 227.33 kJ/kg and 50.00%, respectively. On the other hand, the open loop propane regasification system is most cost-effective for an LNG price between 1.32 and 11 USD/MMBtu. The vast amounts of destroyed exergy in the regasification process of current systems was also demonstrated and hence the need to develop new, more efficient configurations that could exploit the cold energy of LNG.

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The present study provides an energy, exergy and economic analysis of a seawater regasification system (open loop) combining stages of simple organic Rankine cycles (ORCs) arranged in series with an open organic Rankine cycle (OC) in order to exploit the cold energy of liquefied natural gas (LNG). The proposed system, termed ORC-OC, is implemented in a Floating Storage Regasification Unit (FSRU) to achieve the objective of zero greenhouse emissions during the regasification process. Configurations of up to three stages of ORCs and the use of zeotropic mixtures of ethane/propane and n-butane/propane as working fluids are considered in the study of the novel regasification system. Only the two-stage ORC-OC (2ORC-OC) and three-stage (3ORC-OC) configurations accomplish the objective of zero emissions, attaining exergy efficiencies of 61.80% and 62.04%, respectively. The overall cost rate of the latter, however, is 20.85% greater, so the 2ORC-OC results as being more cost-effective. A comparison with conventional regasification systems installed on board shows that the 2ORC-OC yields a lower total cost rate if the LNG price exceeds 8.903 USD/MMBtu. This value could be reduced, however, if the electrical power that exceeds the FSRU’s demand is exported and if compact heat exchangers are implemented.